This invention relates to wireless telecommunications and, more particularly, to controlling the received traffic channel power of a mobile station in a wireless telecommunications system.
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. CDMA techniques utilize uniquely assigned orthogonal codes to spread digitized data over a particular frequency spectrum. The orthogonality of the codes enables multiple mobile station users to overlay their data on the same frequency spectrum. Each overlaid transmission adds to the noise within the frequency spectrum. Without a priori knowledge of the spreading codes used, the collective transmissions appear as noise to a receiver.
Typically, CDMA systems are organized in a cellular structure. That is, a base station which handles mobile station communications for a particular area is centrally located in the area. All of the mobile stations in the area served by the base station transmit and receive data in a manner controlled by the base station. This cellular architecture creates a detrimental effect known as the xe2x80x9cnear-farxe2x80x9d effect that is not found in other forms of wireless communication. The near-far effect is a result of the overlaying of transmissions in the same frequency spectrum. It occurs when a mobile station""s transmissions, as received by a base station, are more powerful than the other mobile stations being serviced by the base station. The more powerful mobile station may tend to drown out the signals of mobile stations which are less powerful or farther away from the base station.
To alleviate the near-far effect problem in CDMA communications, many systems use a power control system to control the transmissions of the mobile stations. Such systems are typically divided into three mechanisms, or xe2x80x9cloopsxe2x80x9d: open-loop, closed-loop, and outer-loop power control. Open-loop power control in CDMA systems consists of an adjustment of mobile station transmission power based on transmission power received at the base station. Open-loop control in CDMA systems provides for a coarse estimate of shadowing. Closed-loop power control for mobile stations in CDMA systems operates by comparing the signal quality from a mobile station, as received at the base station, to a predetermined threshold. Based on the comparison, the mobile station is commanded through a feedback mechanism to increase or decrease its transmission power at predetermined increments.
Outer loop control mechanisms are responsible for maintaining the robustness of the control mechanism. The outer loop control adjusts the system parameters by which the closed-loop controller operates. Thus, in a CDMA system, the outer loop power control mechanism is responsible for maintaining the proper base station threshold signal quality level by which the closed-loop mechanism operates.
In IS-95 CDMA systems, a fast power control procedure is implemented on the reverse link. Third generation digital personal communication systems are being proposed that will achieve higher data rates than the current IS-95 systems. These systems use a bandwidth equal to or wider than 1.25 MHz. All of the currently proposed third generation systems include some form of fast forward link power control command to control base station transmission power. For example, the proposed cdma2000 system uses fast closed loop power control on the forward link dedicated channels. The closed loop system updates at a rate of 800 Hz. A power control update rate of 800 Hz is capable of mitigating fading in mobile stations moving at slow (approximately 3 km/h) to medium (approximately 80 km/h) speeds thereby reducing the required base station transmit power and increasing overall system capacity. The closed loop power control compensates for medium to fast fading and for inaccuracies in open loop power control. Furthermore, fast forward link power control is effective for adaptation of dynamically changing interference conditions due to the activation and deactivation of high power, high data rate users.
To implement power control under the current cdma2000 proposal, a forward fundamental channel (F-FCH) is transmitted at a variable rate, as in TIA/EIA-95-B. When a F-FCH is assigned, power control bits for reverse link power control are punctured onto the F-FCH prior to Walsh spreading. Forward link power control is accomplished using the punctured power control bits to estimate the received forward traffic channel power level. In a cdma2000 system with chip rate three times that in IS-95, for a power control update rate of 800 Hz, 1 symbol out of 12 on each carrier should be punctured for a multicarrier option, 3 consecutive symbols out of 36 for the direct spread 1 antenna case, and 1 symbol out of 18 on each antenna for the direct spread 2 antennas case. Only a very small portion of the forward link total transmitted power is used as the punctured power control bits are transmitted at the same power level as that of the traffic data bits. It remains questionable as to whether the punctured power control bits of this currently proposed method can provide sufficient energy for accurate estimation of channel condition for forward link power control.
The disclosed embodiments provide a method and system for providing efficient transmission power control in a base station. In a third generation IS-95 CDMA network, for example, cdma2000, the forward pilot channel, which is much stronger than that of a traffic channel in an IS-95 system, is used to estimate the received signal power level. In the presently preferred embodiment, a mobile station measures the received pilot channel power, on a common forward link pilot channel. The received pilot power is usually measured in the dB scale. The loss experienced on the pilot channel is estimated as the difference between the pilot channel power transmitted at the base station and the pilot channel power received at the mobile station. The pilot power transmission power is typically fixed based upon the operating environment of the base station. The base station operating environment can vary depending on cell architecture, e.g., macro and micro cells, topography, and other factors. The fixed pilot channel transmission power can be transmitted via a message from the base station.
Based on the estimated channel loss, the received traffic channel power is calculated as the difference between the initial traffic channel transmission power, that is, the power when the mobile station is assigned a traffic channel, and the estimated loss added to any power control gain corrections that have occurred. The initial traffic channel transmission power can be a default value or can be assigned by the base station using signaling messages, e.g., a forward common control channel (or F-CCCH) message.
Power control decisions are made based on the ratio of received traffic channel energy with an estimate of interference and noise on the channel. One method of calculating the received traffic channel is to multiply the received traffic channel power by the duration per chip (or data unit). A xe2x80x9cpower control decisionxe2x80x9d is made by comparing the ratio with a predetermined threshold to determine if the power level should be raised or lowered.
The disclosed embodiments can provide several advantages. For example, the pilot channel is transmitted continuously at a much stronger power level than that of the punctured PC bits. Therefore, more energy can be collected from the pilot channel when estimating the channel condition, particularly pilot channel loss. The traffic channel energy estimate is also improved under the same interference conditions because of the greater amount of collected signal energy. An improved traffic channel energy estimate results in a power control command which can compensate for changes in the channel condition more effectively. The result is that less transmission power is required to achieve the same performance with less interference being experienced by other user. Thus, system capacity can be increased. For another example, use of the pilot channel for forward link power control does not require the extraction of punctured power control bits. Instead, it requires knowledge by the mobile station of only one parameter when compared to the punctured bit approach described above. That is, the disclosed embodiments require knowledge of the transmission power level of the traffic channel when it is assigned to the mobile station.